SYNTHESIS OF LABELED [N-15(3)]-6-BROMOPURINE, A USEFUL PRECURSOR OF N-15-LABELED O-6-ALKYLGUANINES, TO BE USED AS INTERNAL STANDARDS FOR QUANTITATIVE GC-MS ANALYSES

[N-15(3)]-6-Bromopurine was synthesized using readily available labell ed starting materials. The N-15-labelled precursor guanidine, obtained from (NH3)-N-15 and cyanogen bromide, was condensed with ethyl cyanoa cetate to give 4-hydroxy[N-15(2)]diaminopyrimidine. An additional N-15 isotope was incorporated by nitrosation with N-15-labelled sodium nit rite. After reduction to the corresponding triaminopyrimidine and cond ensation with formamide, [N-15(3)]guanine was obtained in 96% yield an d then converted to [N-15(3)]-6-thioguanine by reacting it with phosph orus pentasulfide. [N-15(3)]-6-Thioguanine was readily converted to th e corresponding bromopurine using bromine in the presence of methanoli c hydrobromic acid. The final product was characterized by mass spectr ometry. [N-15(3)]-6-Bromopurine was the precursor for the subsequent s ynthesis of a series of O-6-alkylguanines which can be used in isotope dilution mass spectrometry.